I've been on melatonin for a very long time. Maybe 10-15 years.
It took a very long time for it to not work as well as it used to, but that finally happened.
I find that even with poor sleep, you get more REM and more "recharge" with a healthy dose of melatonin. As far as sleep goes, in my mind it's a miracle drug.
I hadn't heard about melatonin until a thread on reddit where everyone was raving about it and sharing stories about how it knocks them right out. Now I ocassionally take a pill, but I don't really feel any effects and I'm still not entirelly sure if it is putting me to sleep faster or not. I still take it at least for the placebo effect and because it's fairly cheap.
My doc said it could take a week or two initally to have a real effect, ive just passed the 2 week mode, and im combining this with sleeping naked(sorry tmi i know, but hey, it helps haha), and lowering my evening caffeine ammount (i useto only have one no later than 7pm, now i make that one weak).